Optical fiber networks lie at the core of modern telecommunication systems and infrastructures. Employing optical fibers for transmitting information requires particular care to ensure efficient propagation and reception of optical signals. In this regard, the quality and cleanliness of optical fiber connectors represent important factors for achieving adequate system performances. Optical fiber connectors are key optical components that interconnect the network elements by allowing light to travel between two fibers or between one fiber and another optical component with minimal losses and without requiring splicing. Any contamination or damage on the mating surface of an optical fiber connector may severely degrade signal integrity. Since connectors are susceptible to defects that are not immediately discernible by the naked eye, the development of reliable and accurate inspection tools constitutes an important technological challenge.
Poor quality mating of optical fiber connectors arises most frequently when, for at least one of the connectors, contaminants or defects are present on the endface of the ferrule or, yet more deleteriously, on the endface of the optical fiber enclosed therein. The ferrule and enclosed optical fiber correspond to the end portion of a connector. The ferrule protrudes from the connector body in order to be inserted into a coupling device, such as a bulkhead adapter. The ferrule is generally a long, thin cylindrical structure, typically made of glass, metal, ceramic or plastic, that is concentrically bored along its longitudinal axis. This longitudinal bore defines a space for receiving the end of an optical fiber. The ferrule supports the end of the optical fiber and acts as a fiber alignment mechanism when interconnecting the fiber to another fiber, a transmitter or a receiver. By way of example, FIGS. 1A and 1B (PRIOR ART) show a schematic representation of an optical fiber 88 mounted into the ferrule 80 of an optical fiber connector 32 (FIG. 1A) and of a typical bulkhead adapter 82 defining a connector alignment sleeve 84 for mating the optical fiber 88 to another optical device (FIG. 1B).
Upon insertion of the optical fiber inside the ferrule, the end portion of the optical fiber extends slightly beyond the endface of the ferrule. This excess length may then be trimmed off for termination and polished so that the resulting endface of the optical fiber is substantially flush with the endface of the ferrule. The endface of the ferrule is also typically polished, generally to a spherically-shaped surface having a radius of curvature between about 7 mm and 25 mm for ultra physical contact (UPC) connectors. Alternatively, ferrules of some connector types may be polished at a radius of curvature between about 5 mm and 12 mm and at a mean angle having a normal at 7 or 8 degrees with respect to the longitudinal bore of the ferrule into which the end part of the fiber has been inserted. Such connectors, commonly denoted as angled physical contact (APC) connectors, may significantly reduce the degree of back-reflection or optical return loss (ORL) of an unmated connector with respect to the −14-dB ORL of an unmated connector for which the ferrule has not been polished to such an angle. In addition, when connected together inside a coupling device such as an adapter, mated APC connectors generally exhibit a lower residual ORL than their non-angled counterparts.
It will be understood herein that throughout this specification, the expression “endface of the ferrule” is intended to refer to the common end surface of both the ferrule and the fiber. It will also be understood that maintaining good conditions at the endface of the ferrule is important for minimizing optical losses and the generation of excessive ORL at the interface between two optical fibers.
Several inspection techniques and devices, suitable for field use in deployed optical network, have been devised to evaluate polish quality and cleanliness of connectors and other fiber terminations, most notably fiber-optic microscopes and portable video fiber inspection probes.
Fiber-optic microscopes having an adapter to hold the connector in position and a light source for proper illumination can be used as inspection tools for optical fiber connectors. Various fiber-optic microscopes have been developed for such applications, ranging from simple and inexpensive designs to systems having more sophisticated capabilities and options. However, since the ferrule (male) of the optical fiber connector to be inspected is inserted at one end of the instrument and the user must look in at the other end, fiber-optic microscopes are not designed for inspecting (female) connector bulkhead adapters located in patch panels.
Portable video fiber inspection probes constitute another option for evaluating optical fiber connectors. Unlike fiber-optic microscopes, video fiber inspection probes facilitate the inspection of hard-to-reach connectors located on patch panels and bulkhead adapters. These portable probes are equipped with a light-emitting diode (LED) light source for illuminating the connector, a recording element, including for example a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) array, for capturing digital images, as well as adapter tips for most connectors available on the market. Image display can be realized by connecting the probe to an external video screen, a personal computer, or a handheld monitor for fast and efficient inspection.
The above-mentioned prior art is based upon detection of only the spatially-resolved optical power of the imaged ferrule endface, which is directly proportional to the square of the optical electric field amplitude. Such approaches may be termed “direct detection” methods, since they are only sensitive to the optical power, that is, they are not sensitive to the phase of the optical electrical field.
More detailed information leading to improved ferrule endface inspection and characterization may be gleaned by measuring both the phase and amplitude of the light providing the image. An example of such an approach employs “white light” Michelson interferometry, generally in tandem with a “direct-detection” microscope, and the resulting images are displayed on a video screen. Such an approach may be denoted as an interferometric approach, for which the interference occurs between two recombined light beams, originating from a common source, where one of the beams has reflected from the mating surface under inspection and the other beam has reflected from a reference surface. An example of such a method is taught in U.S. Pat. No. 5,459,564 to Chivers, and embodied in the ZX-1 mini-PMS+ product (trade name) commercialized by DORC® Instruments, Inc. The interference fringes produced by such a Michelson interferometer may provide information related to the geometry of the polished surface (e.g. radius of curvature, apex offset, and the like) as well as to the identification of surface defects. However, the design is inherently bulky and unwieldy, thus rendering such an instrument impractical for portable testing in an optical network.
Inspection probes currently deployed for use in the field have the drawback that the accuracy and reliability of the measurements made with them depend essentially on the operator's ability to correctly focus the instrument. When proper focus is not achieved, the analysis software inside the instrument or provided in an external processing unit may not be able to compensate for operator mistakes in adjusting the focus of the device.
Moreover, when multi-fiber connectors, such as MTP, MT-RJ, and MPO types, need to be inspected with a “direct-detection” microscope, each fiber comprising the multi-fiber connector needs to be disposed in the focal plane and within the numerical aperture of the microscope objective by suitable mechanical displacement means. Examples of such mechanical displacement means are taught in U.S. Pat. No. 6,751,017 to Cassady and U.S. Pat. No. 7,336,884 to Zhou et al. A drawback of such approaches is that they involve time-consuming manual adjustment by the user, and a separate image usually must be acquired for each fiber in the multi-fiber connector.
In light of the above, there therefore exists a need in the art for an improved portable inspection probe suitable for the inspection of mating surfaces of optical fiber connectors, which provides fast and reliable performance and that alleviates at least some of the above-mentioned drawbacks.